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I have seen (and recently bought) entry-level rear derailers from Shimano (Tourney for MTB, 2300 for road bike), and I keep wondering why they are making these parts with such large pulleys.

I don't think this make the parts cheaper to produce, since large size means more material, and I can't see why customers would find this more attractive or adequate than the smaller-sized ones (unless customers would prefer more expensive models because they DON'T HAVE large pulleys...).

Also, I imagine there might be advantages to larger pulleys (less chain link rotation, less wear, slower pulley rotation), but the reason why entry-levels are big, while more expensive models continue to be the same old size, is very unclear to me, not to say arbitrary.

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As far as I remember they have not only bigger pulleys but also the distance between them is shorter, isn't it? If this is true, this could be also part of the "problem". –  Benedikt Bauer Feb 3 '13 at 23:18
    
It almost certainly has to do with cheaply filling some niche, probably on low-end mountain bikes. One possibility is that the larger jockey wheels make shifting more reliable with some system, possibly their electric shift scheme. –  Daniel R Hicks Feb 3 '13 at 23:24
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My guess would be better wear on the cheepy bushings from slightly slower rotation. –  alex Feb 3 '13 at 23:50
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Cheaper materials might need to be made bigger and thicker to be as strong as higher end alternatives? Minaturisation needs to be quite expensive. –  Unsliced Feb 4 '13 at 8:39
    
It is known that lower end parts are supposed to last less than higher end ones. That said, and considering the programmed obsolescence common in the bike industry, it would be surprising that such care is taken for the cheaper parts to last LONGER. On the other hand, the last-longer hypothesis makes a lot of sense, and if they are actually doing this it's a good thing. –  heltonbiker Feb 4 '13 at 12:09
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4 Answers

up vote 2 down vote accepted

There are 2 versions of derailleurs which come with extra large pulleys.

The first, which is what is being discussed here, are the low end mega-range compatible derailleurs. A mega-range freewheel has one distinct characteristic which requires compatible derailleurs to have very large cogs.

As seen in the image below, there is a large tooth count difference between the second to largest cog and the largest cog.

Megarange frewheel

This large difference in size requires a large derailleur pulley, to make sure that the chain has cleared the teeth on the last cog, in order to shift into this gear.

If you use a smaller pulley on the derailleur, you may be able to get it to shift, but it will be clunky and loud at best. At worst, it will trap the chain against the side of the largest cog, and refuse to shift into that gear at all.

The second type of large pulley derailleur is decidedly not low end. Berner derailleur

The Berner Carbon Fiber derailleur upgrades are designed to reduce resistance on the drive-train by decreasing the severe chain bends associated with smaller cogs. While expensive, my experience has been an average 10 watt reduction in energy used by the cyclist, with no apparent loss of efficiency in shifting, or in anything else other than the cyclist's wallet.

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Very interesting answer... Let's wait to see what will be the predominant trend for high-end derailers in the future, but I'm starting to think I don't like small pulleys anymore, after all... –  heltonbiker Mar 2 '13 at 13:47
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The main reason for the larger pulleys is that you don't need to make them as strong. More teeth helps to spread the load.

A small pulley is lighter and can aid shifting, but needs to be made from stronger material so you don't shear/wear teeth as much, and so you don't wear the bushings as fast (as @alex commented)

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Why smaller pulleys should be more efficient? I thought it was the opposite! –  heltonbiker Feb 4 '13 at 12:10
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Yeah, I suspect that the larger pulleys would have lower friction and be more efficient, aside from the negligible weight difference. –  Daniel R Hicks Feb 4 '13 at 12:21
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I understand that most friction is in the chain than in the pulley. Smaller diameter pulleys make the chain flex more. That's why, for example, 22x11 is a much less efficient gear ratio than the equivalent 44x22. –  heltonbiker Feb 4 '13 at 12:33
    
Actually, most of the chain-related friction would be in the chain wrapping and unwrapping the front and rear cogs while under tension. But the chain is under very little tension while going through the derailer, and hence friction is low. –  Daniel R Hicks Feb 4 '13 at 16:57
    
Friction in the derailleur pulley is not low, in general. That is not because of chain tension, but rather because of the low end engineering used to make the pulleys. Larger cogs do decrease resistance by decreasing the angles required to wrap and unwrap the chain, and there are products made solely to fix that problem. –  zenbike Mar 2 '13 at 3:10
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Low end drive-trains tend to have a bigger range from smallest gear to largest. Meaning the biggest cog in the cog set on the rear wheel tends to be much bigger. that means there is extra chain length needed to cover all the possible cog/chain ring combinations. (aka the gear you are in) The larger pulley wheels take up extra chain length when not needed.

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I haven't seen any cog larger than 34 teeth, and this cog size is found either in lower end cogs with fewer sprockets (mega-range in a 7 or 8 gear clusters) and in higher-end, multi-geared cassette with 9 or 10 speeds. But the ability to better take up extra chain length is a whole different (and apparently correct) view on the issue... –  heltonbiker Feb 4 '13 at 16:05
    
This definitely does not sound correct to me –  Carson Reinke Feb 10 '13 at 13:30
    
While the pulley size does require more chain, it does not allow a larger cog because of that. The extra chain is taken up by the pulleys themselves. –  zenbike Mar 2 '13 at 3:06
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Low end bikes are often sold to people that are not conditioned athletes. They need very low gears for easy pedalling especially going up hills. The large Cogs provide those low gears. High end bikes are targetted for good athletes that normally would not need those very low gears. Because the low end bikes must cover a large range of sprocket sizes the derailleur cannot be of the small, "short arm" variety.

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The derailleur pulley size does not change your gearing. –  zenbike Mar 2 '13 at 2:43
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